Recently, in 3GPP (Third Generation Partnership Project), “IMT (International Mobile Telecommunications)-2000,” which is the world standard of the third generation (3G) mobile communication system designed by ITU (International Telecommunication Union), has been standardized. “LTE (Long Term Evolution),” which is one of data communication specifications designed by 3GPP, is a long-term advanced system aiming at IMT-Advanced of the fourth generation (4G) and also referred to as “3.9G (Super 3G).” LTE is a communication method based on an OFDM modulation method and OFDMA is used as a downlink radio access method.
In LTE, two duplex communication methods of FDD (Frequency Division Duplex) and TDD (Time Division Duplex) can be selected. In FDD, an uplink coverage area and a downlink coverage area are used. A radio frame format composed of consecutive 10 subframes is used in the uplink and the downlink, respectively. Here, the uplink is communication from a terminal station (UE terminal: User Equipment) to an LTE base station (eNodeB: evolved Node B), and the downlink is communication from the eNodeB to the user equipment. Also in TDD, a radio frame format composed of consecutive 10 subframes is used. In TDD, however, uplink and downlink communications are performed in the same band. Each subframe constituting the radio frame is composed of control signal PDCCH (Phy Downlink Control Channel) from eNodeB and PDSCH (Phy Downlink Shared Channel) used as user data.
Further, LTE performs operation in which a single frequency is commonly used in all cells in a one-cell reuse system. This is because it may cause a shortage of frequency resources when different frequencies are used between adjacent base stations like conventional cellular systems. In this case, there is a problem that radio wave for transmitted and received by user equipment existing near the cell may interfere with each other. Then, in LTE as 3GPP Rel-8, a technique called Inter Cell Interference Coordination (ICIC) of Rel-8 is employed.
ICIC is realized by a fractional frequency reuse in a combination of one-cell frequency reuse and multi-cell frequency reuse, for example. In fractional frequency reuse, each cell can be divided into a center region inside of the cell close to eNodeB and a peripheral region at a cell edge away from eNodeB. A “central frequency” allocated to communication between eNodeB and a user equipment in the center region is competitive with adjacent cells (that is, one-cell frequency reuse); however, interference between cells can be avoided by maintaining transmission power small so that signals can be transmitted only in the center region. On the other hand, although large power is required to transmit signals to the peripheral region, interference between cells can be avoided when different “peripheral frequencies” are used in peripheral regions of the adjacent cells (that is, multi-cell frequency reuse).
FIG. 24 illustrates a manner that three cells 1 to 3 that perform the fractional frequency reuse are adjacent to one another. In the figure, the hexagonal shape represents a single cell coverage. The cells 1 to 3 are divided into center regions illustrated as blank parts inside the cells and peripheral regions illustrated as hatched parts in the periphery of the cells. Although the central frequency allocated to the center regions is competitive with adjacent cells (that is, a single frequency reuse), interference between cells can be avoided by maintaining transmission power small so that signals are transmitted only in the center region. On the other hand, different frequencies are allocated to peripheral regions of the adjacent cells. In FIG. 24, differences of frequency band are represented by hatching types (diagonal hatching lines, vertical hatching lines, and horizontal hatching lines).
Further, in Rel-8 ICIC of 3GPP, in addition to the above described frequency reuse technique, signals for suppressing interference are exchanged between base stations, which are eNodeB, via an X2 interface (The X2 interface is an interference connecting between eNodeBs and an optical fiber is its typical example). As messages exchanged via the X2 interface, concretely, there are a High Interference Indicator (HII) and an Overload Indicator (OI). Here, an HII is information to inform a location of a resource block allocated to the user equipment at cell edges to adjacent eNodeB. From the resource block specified by the HII can be determined to have a high possibility to receive interference. Thus, considering the possibility, scheduling is performed on the resource block in the adjacent cells. On the other hand, OI is information to inform a degree of interference to the uplink resource block and has three levels of low, medium, and high. When it is informed that the degree of interference to a certain resource block is high by OI via the X2 interface, an adjacent eNodeBs adjusts scheduling and an uplink power control regarding the resource block.
In this manner, Rel-8 ICIC has an object to remove interference between macro cells and employs a method to adjust via the X2 interface. However, what is adjusted by this method is only PDSCH in a subframe and PDCCH part cannot be adjusted. This is because PDCCH is a format in which adjacent cells use the same frequency band to resist interference.
Next, Rel-10 ICIC will be explained. Rel-10 ICIC has an object to control interference between a macrocell and a picocell.
In 3GPP, a method called HetNet that improves a capacity of an entire system by layering cells in various sizes such as macro, micro, pico, and femto. Pico eNodeB, which is a base station of a picocell, has a characteristic of having transmission power lower than that of Macro eNodeB as a base station of a macrocell by several dozens dB. It can be assumed that there is an X2 interface between Macro eNodeB and Pico eNodeB (in other words, interference to the PDSCH part in a subframe is already solved by Rel-8 ICIC). Here, it has to be assumed that the X2 interface between Pico eNodeB and Macro eNodeB is an interface having a weaker characteristic in speed, capacity, and delay than the X2 interface between Macro eNodeBs in some cases.
Since the transmission power from Pico eNodeB is low, there are many areas in which signals from Macro eNodeB can be strongly received. Even in an area in which propagation loss from the picocell is smaller than propagation loss from Macro eNodeB (or, even in an area being closer to picocell than Macro eNodeB), when received power from Macro eNodeB is larger, the user equipment may often attempt RRC (Radio Resource)_Connected to further Macro eNodeB, not to close Pico eNodeB. However, since it is advantageous to connect to a base station having smaller propagation loss in an uplink in view of battery consumption of a terminal and it is important to obtain cell division gain by allocating user equipment to the picocell in HetNet environment, it is required to solve a problem that user equipment is made to connect only to Macro eNodeB.
Then, Rel-10 specifies a technique called a range expansion. The range expansion will be explained with reference to FIG. 25. When selecting a cell, that is, determining a base station to enter, the user equipment selects to enter eNodeB which has larger power based on received power (RSRP: Reference Signal Received Power) obtained from a reference signal (cell-specific reference signal) from eNodeB. When evaluating RSRP of each eNodeB, an area where the user equipment entering Pico eNodeB exists is enlarged by adding an offset of 10 dB to RSRP of Pico eNodeB to evaluate, for example. This is the range expansion and the enlarged area is called a range expansion area. The range expansion area is an area in which RSRP offset, that is a technique of the range expansion, allows the user equipment to enter Pico eNodeB even in a case that the user equipment generally enters Macro eNodeB due to the low RSRP from Pico eNodeB.
For the user equipment existing in the range expansion area, received power from Macro eNodeB is sometimes greater than received power from entered Pico eNodeB. In other words, in the range expansion area, there is a disadvantage that reception in the user equipment from Pico eNodeB is weak against interference from Macro eNodeB. This is why, in the range expansion area, interference in the downlink between Pico eNodeB and Macro eNodeB becomes a problem.
For example, there has been proposed a communication system including a base station device for managing a mobile station device and a macrocell, and a home base station device for managing a femtocell, a picocell, a nanocell home cell, and the communication system adjusts interference to home base station device (see Patent Document 1, for example). However, this communication system does not adjust interference in the downlink to a user equipment in a range expansion area.